An automatic transmission fluid (ATF) must fulfill special requirements because of the conditions of its use. During normal use, automatic transmission fluids can be exposed to extremes of low and high temperatures, i.e. temperatures which can range from about -30.degree. C. to +175.degree. C. Therefore, essential features of an ATF include a low viscosity at a low temperature, a high viscosity index (VI) and a low pour point.
The measurement of viscosity indicates a fluid's resistance to flow which tends to decrease as the temperature increases and increase as the temperature decreases An ATF, under normal operating temperatures (i.e., 80.degree. C. to 120.degree. C.) must have a sufficiently high viscosity to provide adequate lubrication. However, when the temperature increases, it must retain sufficient viscosity for adequate film and wear protection. As temperatures decrease, it must maintain proper fluidity.
Viscosity Index (VI) is an indicator of how fast a fluid decreases in viscosity, or thins, as the temperature increases and the amount of thickening, or increase in viscosity, which occurs as the temperature decreases. VI is determined from the kinematic viscosity of a fluid based on known viscosity-temperature relationships.
However, knowing the VI does not always help predict the lubricant viscosity at temperatures below about -5.degree. C. At these lower temperatures, the viscosity of even very high VI oils can deviate from the expected viscosity as predicted from the known viscosity-temperature relationship. Thus, when an oil is described as possessing a high VI it is not entirely clear that this oil will have the necessary low viscosity at low temperatures required for ATFs.
A special test called the Brookfield viscosity test (ASTM D2981) has been developed to evaluate low temperature viscosity performance, as pointed out in Schodel "Automatic Transmission Fluids (ATFs)-The Improvement of Low Temperature Characteristics", 47 Lubrication Engineering pp 463-467 (June 1991). The Brookfield viscosity test measures the viscosity of fluids over temperatures ranging from -5 to -40.degree. C.
To achieve low pour point and good viscosity properties for ATF basestocks, refiners have considered solvent extraction followed by catalytic dewaxing of neutral distillates to produce 100 SUS oils. However, this has only achieved moderate success as the lubricants can suffer from low viscosity and VI.
Although high severity catalytic dewaxing of slack waxes can meet the very low pour point requirements, this is a costly proposition.
Upgrading crude fractions of low quality, such as high asphaltene, resinous fractions, for lubricant manufacture can be performed by hydrocracking, sometimes referred to as "severe hydrotreating". In this process a fraction from the upper portion of the lubricant boiling range is catalytically reacted with hydrogen under pressure. The process converts the components which are unsuitable for lubricant performance to those which are suitable. A substantial fraction of the polynuclear aromatics is hydrogenated and cracked to form naphthenes and paraffins. Process conditions and choice of catalyst are selected to provide an optimum conversion of the polynuclear aromatic content of the stock since this component degrades the viscosity index and stability of the stock. The paraffins are isomerized to impart good VI characteristics to the final product.
Hydrocracking to produce conventional base stocks from marginal crudes has been carried out as a stand-alone process in which the feed is directly subjected to the hydrocracker without preliminary solvent extraction. The hydrocracking process has greater feedstock flexibility and gives higher yield and quality compared to solvent refining. The disadvantages, however, include higher capital cost, lower basestock viscosity and lower aromatics content which may require reformulation to aid in additive solubility. Also, although hydrocracking improves product VI, hydrocracked stocks generally have poor seal compatibility, an unacceptably high pour point and must be dewaxed by solvent or catalytic dewaxing.
Processes which combine solvent refining and hydrocracking have been proposed with the goal of retaining the high product yield of standalone hydrocracking while maintaining the viscosity properties and lower processing costs of solvent refining.
U.S. Pat. No. 4,622,129 describes a process for making lubricating oils from nitrogenous distillates and/or deasphalted oils by the solvent extraction and hydroprocessing of a wide variety of crude oils. A more valuable lubricant product, in high yield and of consistant product quality is proposed. The base oils after solvent dewaxing are described as possessing pour points below about -9.degree. C. and VI of about 95. There is no indication of the low temperature viscosity properties necessary for a basestock to be used in automatic transmission fluids and from the VI, the product has not met the VI requirements.
United Kingdom patent No. 1240913 describes a modification in lubricant processing by solvent extraction and catalytic hydrotreating to achieve a high VI product. The described chargestock covers a wide range of petroleum fractions having a VI less than 90 and the proposed solvent extraction and hydrorefining conditions applicable to this range of feeds is equally as broad. There is no description of the low temperature properties of the product. A wax distillate fraction (VI of 70) is subjected to furfural extraction to a VI of 107 and pour point of +105. The raffinate is then mildly hydrorefined at a temperature of 775.degree. F. and pressure of 2,500 psig and solvent dewaxed to produce a lubricant having a VI and pour point ranging from VI 101, 0.degree. F. pour point, to VI 110 and -20.degree. F. pour point, depending upon the space velocity. Although the pour point is good, and a VI of 101 is not considered low, for purposes of ATF performance, better properties would be a significant advance in basestock production for automatic transmission fluids. Also, there is no indication that the lubricant product would meet the low temperature Brookfield viscosity specifications essential for an ATF.
United Kingdom patent application No. 2059433 describes another attempt to improve the VI of a paraffinic raffinate to achieve a base oil suitable for manufacturing a high performance industrial oil. The base oil is made by mild extraction and mild hydrocracking conditions of temperature ranging from 375-450.degree. C. and pressure ranging from 30 to 100 bars (435 to 1,450 psig). The properties of the chargestock are defined in terms of the aromatics content which range from 10 to 35% by weight. The highest reported VI is 114 with pour points ranging from -9 to -24.degree. C. For purposes of achieving an ATF basestock suitable for purposes of an automotive ATF performance, at minimum additive treat levels, it is preferred for the basestock to achieve a still higher VI, while maintaining a low pour point.
Moreover, industrial oils which are referred to in U.K. 2059433 are not expected to meet the same rigorous performance specifications as automotive oils which are exposed to much more severe conditions.
The automatic transmission is a hydrokinetic unit, converting power from the engine into kinetic energy in the torque converter pump. The engagement of clutches and bands has to be rapid and smooth for good driver shift feel characteristics and vehicle performance. It is this mechanism that automatic transmission companies are concerned with for new designs. As well as focusing on automatic transmission design, automobile manufacturers have looked at ATFs for improving driveability. Certain manufacturers have developed specifications which ATFs must meet, i.e., General Motors DEXRON II and Ford MERCON. The concern for better driveability also prompted General Motors to set the lead for more stringent low temperature viscometric requirements for ATFs, i.e. 20,000 cP (DEXRON IIE) vs. 50,000 cP (DEXRON IID) maximum Brookfield viscosity at -40.degree. C.
By contrast, the most common type of power transmission units used in industrial applications (i.e. manufacturing machines, farm, construction and off-highway vehicles and equipment) are hydrostatic drive transmissions. The hydrostatic transmission consists of a pump and motor as opposed to the torque converter, planetary gears, clutches and bands of an automatic transmission. The DEXRON IIE specifications for industrial hydrostatic transmission fluids have no low temperature viscosity performance specifications and allow the VI to range from 105 to 150, as opposed to the Brookfield viscosity maximum of 20,000 cP at -40.degree. C. and VI of 190 which are required for automatic transmission fluids. Thus, although a VI of 114 is reported in U.K. 2059433, because of the unpredictability of the viscosity properties at low temperatures, success in terms of low temperature viscosity properties would not be expected by utilizing a specific feedstock at more severe process conditions.
U. S. Pat. No. 3,880,747 discloses treating a naphthenic distillate stock boiling above 580.degree. F. with an aromatics selective solvent and hydroprocessing to produce a basestock for automatic transmission use. The broad hydroprocessing conditions include temperatures ranging from about 500.degree. F.-800.degree. F. 0.1-8.0 L.H.S.V., pressure of 500 to 3,000 p.s.i, and hydrogen circulation ranging from 0-20,000 SCF/bbl. There is no description of achieving a high VI oil which has good viscosity properties at low temperatures.
A "naphthenic" distillate, as referenced in U.S. Pat. No. 3,880,747, is described in U.S. Pat. No. 3,839,189. The naphthenic distillate is a vacuum distillate fraction which is preferrably wax-free and has a viscosity-gravity constant (VGC) in the range of 0.820 to 0.899 and a viscosity in the range of 150-12,000 SUS at 100.degree. F. The fraction can be obtained by deep furfural extraction to produce a wax-free low VGC fraction.
These disclosures fail to recognize the advantages of using a medium neutral distillate and high pressure hydrocracking conditions in order to obtain good viscosity properties at low temperatures.